- OC的变量限定词的官方解释：
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__strong is the default. An object remains “alive” as long as there is a strong pointer to it.（当属性没有使用修饰词时，默认的是strong，只要有强指针指向它，这个对象会一直存在）
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__weak specifies a reference that does not keep the referenced object alive. A weak reference is set to nil when there are no strong references to the object. （_weak修饰的对象并不会持有改对象，当一个对象没有强引用的指针指向它时，weak修饰的对象会被制空）
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__unsafe_unretained specifies a reference that does not keep the referenced object alive and is not set to nil when there are no strong references to the object. If the object it references is deallocated, the pointer is left dangling.（__unsafe_unretained 和weak类似，但是当对象没有被强引用时，__unsafe_unretained修饰的对象还会指向该对象的地址）。
